So-called low carbon steel with a carbon content of 0.3% or less is excellent in workability and weldability and is used in many structures. Buildings, vehicles, ships, industrial machinery, etc. use such low carbon steel to form frames, inside partitions, or outer shells. The structural members mainly provide the required strength. In “mild steel” with reduced C, there has been considerable development of technology for raising its strength extremely high to reduce the weight of the structural members. By means of raising the specific strength (intensity per unit mass) of steel, welded structures have achieved larger and more complicated structures and furthermore secured safety though the increase in the strength of steel.
However, to raise the strength of low carbon steel, a large amount of alloy elements other than carbon have to be added. Alternatively, there were the problems that this led to increased complexity of the system for strict control of the crystal structure when producing the steel or otherwise led to a drop in productivity or rise in production cost in return for the higher strength or workability. In particular, in recent years, attempts have been made to reduce the welding process unavoidable at the time of production of structural members as much as possible. Technology increasing the weld input heat has been developed. As a result, the input heat at the time of welding often exceeds 50,000 J/cm. Welding is even being performed with an input heat exceeding 100,000 J/cm in some cases and 200,000/cm in buildings. In the case of such high weld input heat, the welded material receives a large heat effect, is exposed to a high temperature of 1400° C. near the molten metal, and is exposed to a temperature of the A3 transformation temperature of steel, 900° C. or more, that is, the range of the so-called “heat affected zone of weld” is broadened.
As a result, in this heat affected zone of weld, the structure of low carbon steel produced by strictly controlling the structure changes to an uncontrollable form of structure determined by the rise in temperature due to the large weld input heat and the subsequent cooling rate of the weld joint. Whether or not heat treatment of an extent of annealing for removing the residual stress after welding is performed, this heat treatment is not reheating to the transformation temperature or more, so it is difficult to improve the transformed structure.
In such a weld heat affected zone as well, the structural member is required to maintain characteristics similar to those of sound parts not affected by the weld heat. In the final analysis, bringing out the characteristics of the steel material at this heat affected zone of weld becomes the most important issue. Technical development to secure this has mainly focused on development of new steel materials.
An invention relating to a steel material utilizing nitrides or oxides resistant to breakdown at a high temperature so as to prevent an increase in the prior γ grain size in the crystal structure, particularly near the fusion line, is disclosed in Japanese Patent Publication (B) No. 57-19744, Japanese Patent No. 3256118, etc. However, in the high-tensile steel having a strength of 550 MPa or more and having a structure comprised of at least 60% bainite covered by the present invention, even if applying these inventions, the above-mentioned mode at the time of production of the structure performed for bringing out the strength of the material, that is, a grain size equal to that of the matrix, dislocation density, and dispersion density of precipitates, end up changing due to the retransformation caused by the heat effect at the time of welding, so is difficult to reproduced. Even if a toughness equal to that of the matrix can be secured, the problem remains that it is difficult to obtain a strength at such a heat affected zone equal to that of the matrix.
On the other hand, the technique of adding Ni, Cr, Mo, etc. to improve the hardenability of the steel material and secure the strength is naturally appropriate thinking in alloy design. However, Ni and Mo are expensive elements. Industrially, adding large amounts of for example over 5% to structural steel is not practical. When limiting the amounts of addition to avoid a large rise in cost, there is less effect. The increase in cost becomes a problem, so this is not a practical solution. Further, Cr easily causes precipitation embrittlement. In exchange for the rise in strength, the toughness has ended up being lost. The same is true even when adding large amounts of Nb, V, Ti, and other elements. When trying to obtain a good balance of characteristics in the heat affected zone of high-tensile steel, industrially a stalemate has been reached in almost all cases.
On the other hand, while the mechanism is not clear, in the technology adding W to improve the strength of steels, numerous technologies focusing on heat resistant steel have been developed. Japanese Patent Publication (A) No. 10-46290, Japanese Patent Publication (A) No. 8-225884, and Japanese Patent Publication (A) No. 9-217146 describe inventions relating to heat resistant steel containing Cr in an amount of 0.8% or more where W is included in an amount of 0.01 to 3.5% for the purpose of improving the creep rupture strength. However, these are all aimed at improvement of the creep characteristic. When it comes to achieving both strength and toughness at the heat affected zone of weld, since the temperature specification of heat resistant steel is even at the lowest 400° C. or more, there is almost no demand for toughness. Even when there is, it concerns cracking at the time of installation or damage at the time of water pressure tests. Further, in high temperature, high pressure power generating plants or petrochemical plants traditionally made of heat resistant steel, welding conditions with high input heat are not employed at all due to the concern over embrittlement of the weld joints. Therefore, W is added not so as to control the characteristics of the heat affected zone of weld due to the input heat. Further, the form of presence in the steel also naturally differs. Securing the properties at the heat affected zone during large input heat welding performed on structural members at room temperature or less covered by the present invention is not considered. Due to the chemical composition, even if applying large input heat welding, as a general rule the toughness inevitably remarkably drops.
Further, as technology for adding W to a structural material used at room temperature or less, there is the example of application for improving the characteristics of other steel materials. Japanese Patent No. 2633743 discloses an invention relating to a method of production of thick steel plate controlling the grain size to a refined size and describes an invention relating to a steel material to which W is added in a range of 2.0% or less. However, in this case, W is added for the purposes of improving the hardenability of the steel material, so there is no description relating to the ratio of precipitation. Therefore, the technology for effectively utilizing solution strengthening is not seen at all.
Japanese Patent Publication (A) No. 4-350119 describes an invention relating to a method controlling the temperature distribution in the transverse direction of the steel plate so as to control the crystal grains in the sheet plane of the steel plate to be uniform everywhere, but even in this case does not describe the technology, when adding W, of limiting the amount of precipitation for the purpose of improving the hardenability. That is, there is no description of the art of positively utilizing the in-grain solution strengthening of W. Similarly, Japanese Patent Publication (A) No. 9-271806 has a description of an invention relating to a method of production of steel plate and steel plate aiming at uniformity of scale at the surface of steel plate, but there is no discovery relating to control of precipitation of W in exactly the same way as the above technologies and positive utilization of solution strengthening is not considered.
Japanese Patent Publication (A) No. 7-331382 and Japanese Patent Publication (A) No. 2003-3229 describe inventions for improving the fatigue strength of a heat affected zone of weld. Only descriptions of adding W in an amount of 0.01 to 2.0% to cause it to act by precipitation strengthening or in-grain solution strengthening are found. However, there is no allusion to the ratio of precipitation. Precipitation as intermetallic compounds is also not mentioned at all. These are aimed at improvement of the strength of the steel material just by adding W. Naturally, when the amount of precipitation is not controlled, even if using these technologies, the goal of the present invention, that is, achievement of a strength and toughness of 550 MPa grade steel at both of the matrix and weld zone is difficult due to the above-mentioned reasons.
On the other hand, a description of an invention simultaneously adding Ti and W and causing W to precipitate to contribute to the strengthening is seen in Japanese Patent Publication (A) No. 2003-313630. However, the technical idea of simultaneously adding Ti and W is also described in Japanese Patent No. 2987735. Further, the method of using heat treatment to control the structure is also described, but breakdown of stable Ti—W—C precipitate etc. should not occur due to its nature. Rather, there is only the finding of defining and making effective use of the amount of precipitation.
Therefore, as explained above, the alloy designs of high-tensile steel in the prior art only relate to inventions using the fact that W always improves the strength of a material by precipitation strengthening as their first actions and effects. In these inventions, inevitably the problems have remained that embrittlement due to precipitates in the heat affected zone of weld is hard to avoid and securing strength and toughness characteristics similar to the matrix at the heat affected zone of weld in a high-tensile steel of a strength of 550 MPa or more was impossible to realize.